1. Field of the Invention
The invention relates to a ticket issuing device for flight or other passenger tickets, each having a magnetic stripe therein, and to be more precise, relates to a reader for magnetic data and a method for reading the magnetic data on passenger tickets.
2. Description of the Related Art
For more efficient and better passenger service provided by airline or other transportation companies, passenger ticket issuing devices have recently been developed, each device being provided with a magnetic data reader which records various information into the magnetic stripes on the passenger tickets, reads data on the magnetic stripes, and performs various kinds of processing based on the data.
In a conventional way of reading magnetic data by using this type of device, data on the magnetic stripe of each passenger ticket are transferred to the fixed magnetic read head, an output from the magnetic read head is amplified to a required analog level, and then the amplified signals are converted to digital signals by a peak sense circuit.
Since the converted digital signals are constituted to have a frequency modulated form (hereinafter referred to as "FM"), they are further converted to binary data by demodulation of the signal. In the FM mode, data having a specified reference frequency F are converted to "0" whereas data having frequency 2F, that is double the reference frequency F, are converted to "1". The value of the reference frequency F is achieved by calculation of a pulse width length of the detected digital signal.
Various required data specific to a particular ticket, i.e. passenger data, in addition to clock data "0"s used for detecting each of the required data, and management data, are recorded in the magnetic stripe of each passenger ticket; the clock data contain a specific number of consecutive "0"s to detect the beginning of the required data. Accordingly, in the detection of data, clock data and start sentinel data, i.e. start bit data, which are defined among the management data stored in advance at a predetermined region of a memory, are compared with clock and start sentinel data read from the stripe, to detect the beginning of the required data. The magnetic data consist of a plurality of blocks, and the clock data and the start sentinel data are put at the beginning of each block. After the required data are detected, they are converted from binary to hexadecimal form of specified bit length and then finally converted to character data.
The reference frequency of magnetic data output from the data medium depends on its transmission speed. The binary data are achieved by calculation of the frequency of data which were acquired just before the data to be read because the frequency of binary "1" is twice as high that of binary "0" in the FM mode. Although it is thought that a predetermined fixed frequency is used, instead of the above, as the reference frequency, it may inherently reduce the accuracy of the calculation in the FM mode. The reference frequency is determined within a certain permissible range on the assumption that some fluctuation may occur in the transmission speed of the data medium. Excessive fluctuation of the transmission speed, however, hardly occurs within one-bit of data in the transmission mechanism. Accordingly, the reference frequency is achieved by calculation of the frequency of the one bit of data which was acquired just before the data to be read.
However, the above-described magnetic data reader has encountered the following problems:
Although the demodulation technique of the conventional magnetic data reader described above assumes that there is no excessive fluctuation of the transmission speed of the data medium, a peak shift may be generated in the output of the read data caused by a perforation on the passenger ticket, a flaw on a magnetic stripe made by a passenger while carrying the ticket, or an interfering noise from adjacent tracks if the magnetic read head is used for reading data from a plurality of tracks in a magnetic stripe. The peak shift may cause a pulse width of the digital signal (a period of time between two states of the digital signal) to be temporarily longer (lower frequency) and the next pulse width of the digital signal to be temporarily shorter (higher frequency) within the permissible range, even if the written data are the same as the previous data.
In the demodulation, this makes the pulse width of read data longer than the previous one within the permissible range in which a demodulation error is not detected. Therefore, the data am set to binary "0", and conversely the subsequent pulse width becomes shorter by as much as the previous extension and then the data are set to "1". This is because the pulse width of only just the previous one bit is used for calculation of a reference pulse width in the conventional demodulation technique. In addition, an erroneous encoding of data such that all subsequent data are set to "1" occurs, because the pulse width whose data were set to "1" is specified as the reference frequency.
If an excessive fluctuation of time (pulse width) is detected in the demodulation circuit, the demodulation circuit is reset to restart the demodulation because demodulation errors are considered to have occurred. However, the permissible range of the pulse width permits fluctuation to some extent, since some degree of fluctuation of the transmission speed of the data medium is assumed. This allows the pulse width of the digital signal to be longer or conversely shorter within the permissible range, which causes a read error based upon erroneous encoding of data, instead of a detected demodulation error, such that read data are set to "1" for written data "0" or conversely set to "0" for written data "1".